An optical waveguide modulator having a field-effect transistor-type electrode using a thiophene oligomer, alpha-sexithienyl (6T), has been fabricated and has functioned as an electro-absorption modulator with the modulation ratio of 20%. The optical modulation is directly related with the FET device operation, and has shown a maximum at 830 rim for propagating light wavelength. Therefore, the mechanism of the optical modulation has been clarified to be the increasing absorption due to polarons induced by negative bias. The linear relationship between the modulation ratios and the FET source currents has led to the conclusion that the polaron conversion efficiency eta was introduced as the new parameter to understand the device operation, which has become clear. From the analysis, the nearly intrinsic value of carrier mobility of 6T was extrapolated, and the limit of the improving the carrier mobility was also shown. The possibility of enlarging the modulation ratio and its frequency dependence is also discussed.